The present invention relates to an apparatus for determining the condition of a fluid in a space, comprising:
a measuring electrode extending inside the space;
a second electrode placed in the space; and
a measuring circuit for measuring the impedance between the measuring electrode and the second electrode and calculating the condition of the fluid from the impedance.
Such an apparatus is known from WO 96/24823.
This known apparatus provides excellent results in measuring the quality, such as the degree of filling or the dielectric constant, of the fluid situated in the vessel. Fluid is herein understood to mean not only a gas or a liquid but also ocher substances behaving as a fluid, such as granulates and substances in powder form.
This known apparatus measures not only the capacity between the measuring electrode and the second electrode but also the ohmic resistance connected in parallel thereto, or the conductivity of the product between the measuring electrode and the second electrode. It will be apparent that the measurement result is influenced hereby.
There is therefore a need for an apparatus for measuring substances by means of other properties, such as the ohmic resistance. This is for instance also the case with substances wherein the dielectric constant differs only little from that of air.
This known apparatus moreover has the drawback that when the dielectric constant of the substance present in the space is not known precisely, the measurement signal is a function of the dielectric constant of the substance and of the degree of filling.
It is of course possible to arrange a separate measuring electrode for this purpose which is always immersed in the substance and which serves to compensate the dielectric constant, although this results in a costly and thus less attractive solution.